In diesel engines installed in automobiles, a plurality of fuel injections are carried out in each cylinder during one engine cycle, with a view to, for instance, reducing the amount of NOx and soot in the exhaust gas, lowering noise and vibration, and improving fuel consumption and torque. For instance, Japanese Patent Application Publication No. 2009-293383 (hereafter, Patent document 1) discloses a fuel injection mode in which two injections, namely a pre-injection for premixing and a pre-injection for pre-heating, are performed ahead of a main injection for torque generation.
Concerning the setting of injection timing in pre-injection for premixing and pre-injection for pre-heating, Patent document 1 also discloses the features of determining the timing at which a heat release amount or a heat release rate drops in a cylinder while varying the injection timing of pre-injection for pre-heating towards the advance side, in a state where the injection timing of the main injection is fixed, to acquire thereby an advance angle limit of pre-injection for pre-heating; and determining similarly the timing at which the amount of heat released in the cylinder becomes zero while varying the injection timing of pre-injection for premixing, in a state where the injection timing of the main injection is fixed, to acquire thereby the lag angle limit of pre-injection for premixing.
In diesel engines installed in automobiles, NVH (Noise Vibration Harshness) performance is an important consideration at a region of high frequency of use and of partial load on a relatively low-speed side (low load to medium load). In particular in the medium load region within the above operating region, the heat release rate (or heat release speed dQ/dθ) increases abruptly accompanying an increase in the fuel injection amount during main combustion at which the fuel injected by the main injection burns (combustion where diffusion combustion is predominant). This is likely to give rise to an increase in combustion noise and loss of NVH performance, all of which is problematic.
As a countermeasure, the technology disclosed in the abovementioned Patent document involves only setting the lag angle limit of pre-injection for premixing and the advance angle limit of pre-injection for pre-heating, and fails to suppress abrupt increases in the heat release rate during the main combustion. Therefore, there is a need for technologies that allow actively controlling the combustion mode in the cylinders so as to suppress abrupt increases in the heat release rate during the main combustion in a predetermined load region.